can't import alembic file

Do anybody have an idea why my Unity (Version 2019.3.0a4) can't import or even find the Alembic/abc files. I already imported my .abc file 2 days ago and everything worked as I wanted. The next time I opened the project, it gave me the error "..failed to load alembic". The next time, it disappeared. So now it says "missing prefab". I already tried to delete this not-working-file and tried everything again. But now when I want to import my abc file it's not even shown, only the .mb file of it and that is useless in this case. I hope anybody has an idea to solve this problem.